High performance achieved via core engineering and side-chain engineering in organic solar cells based on the penta-fused-ring acceptor

Abstract

Recently, low-band gap non-fullerene acceptors (NFAs) have achieved rapid development and proven to be an effective means to improve the performance of organic photovoltaics (OPVs) because of their adjustable energy levels and strong near-infrared (NIR) absorption. In order to enhance light capture in the NIR region, strong electron donating and electron withdrawing units were introduced to enhance intramolecular charge transfer (ICT). However, the commonly used end-group engineering often fails to achieve a simultaneous enhancement of short-circuit current (J_sc) and open-circuit voltage (V_oc). In contrast, the strategy of core engineering can enhance both J_sc and V_oc. Moreover, among five-membered aromatic heterocycles, pyrrole has the highest electron density, and thus multi-pyrrole could be a strong electron donating unit. Therefore, we designed and synthesized a novel pyrrole-based NFA (DPBT-2Cl) for comparison with a thiophene-based NFA (TPBT-2Cl). Red shifts of 55 nm (in solution) and 75 nm (in film) of the absorption peaks were achieved through core engineering by substituting pyrrole for thiophene. The morphology of the active layer also plays a crucial role in the performance. By modifying the N atom of pyrrole with alkyl chain as the side-chain engineering, the phase separation of PM6/DPBT-2Cl in the active layer is more ideal than that of PM6/TPBT-2Cl. The best power conversion efficiency (PCE) increases from 1.0% for TPBT-2Cl to 11.5% for DPBT-2Cl, which is one of the high performances we know of for reported OSCs based on the penta-fused-ring (PFR), with J_sc increasing from 3.56 mA cm⁻² to 23.1 mA cm⁻² and V_oc from 0.74 V to 0.78 V. We demonstrate that core engineering by replacing thiophene with pyrrole enables low-band gap NFAs and side chain engineering enables a more ideal phase separation in the active layer, which is an excellent strategy to realize a simultaneous enhancement in both V_oc and J_sc and maximize the performance of OPVs.